The mrd_position(3mrd) routine provides access to the SCSI
    Position to Element command. For the robot specified by the
    robot_name, the routine will attempt to position the specified
    transport to the specified element. The transport and element
    addresses are zero based. On subsystems that support inverting a
    cartridge during a move, the cartridge_side argument can be used
    to indicate that the transport should be inverted to invert a
    cartridge.

    The robot will be opened and the arguments to the function
    verified that they are appropriate. The element address and
    type will be checked that they are within the valid range of
    elements on the robot. The cartridge_side argument will be
    verified that it is either the value one (1) or two (2). All
    pointer arguments, except transport, are checked to verify they
    are not NULL pointers.

    Many robot subsystems support an absolute transport address of
    zero for the Position to Element command so that the robot can
    select the appropriate transport if multiple are available. This
    routines allows the default transport address to be specified by
    using a NULL pointer for the transport string.